Slip and acceleration of the wheels of an automotive vehicle provide information on wheel lock-up conditions from which brake pressure commands for each of a plurality of brake pressure actuators may be generated in an automotive vehicle antilock braking system ABS. It is known to estimate wheel slip and acceleration using wheel speed information from the wheels of the vehicle. The estimated slip and acceleration may be applied to pressure command functions or as reference indices to predetermined look-up tables from which the brake pressure commands are retrieved.
The information provided by the estimated wheel slip and acceleration is limited to indicating whether wheel lockup conditions are present, and does not distinguish the source of the estimated wheel slip and acceleration leading to the indicated lock-up condition. Conventional antilock braking control systems treat a wheel lock-up condition as a low driving surface friction coefficient .mu. condition, as such systems don't have information on tire normal force Fz. For example, when a large decrease in tire normal force occurs, the corresponding wheel may lock up due to a lack of frictional force Fx, wherein frictional force Fx may be expressed as EQU Fx=.mu.*.lambda.*Fz
where .lambda. is wheel slip. This can lead the conventional antilock system to interpret a presence of a low-.mu. driving surface, since it assumes a frictional force change is a result of a friction coefficient change. As a result of the interpreted presence of a low .mu. driving surface, the antilock brake control algorithm will command a brake pressure reduction to avoid lockup. This reduction may be unnecessary however, as the decrease in normal force is typically only transient in nature, and the wheel speed will thereafter recover on its own. Such unnecessary brake pressure reduction can result in false entry into antilock braking control and decreased braking performance.
Accordingly, what is needed is an antilock braking control approach that can better distinguish and react to the source of a change in tire frictional force, by including tire normal force information in the control of braking force so as to sustain braking performance under uneven driving surface conditions.